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          canvas-绘制圆和矩形
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        <h3 id="绘制圆"><a href="#绘制圆" class="headerlink" title="绘制圆"></a>绘制圆</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> canvas = <span class="built_in">document</span>.getElementById(<span class="string">&#x27;canvas&#x27;</span>)</span><br><span class="line"><span class="keyword">const</span> ctx = canvas.getContext(<span class="string">&#x27;2d&#x27;</span>)</span><br><span class="line">canvas.width = <span class="number">600</span>;</span><br><span class="line">canvas.height = <span class="number">600</span>;</span><br><span class="line"></span><br><span class="line">ctx.beginPath();</span><br><span class="line">ctx.arc(<span class="number">300</span>, <span class="number">300</span>, <span class="number">50</span>, <span class="number">0</span>, <span class="number">2</span>*<span class="built_in">Math</span>.PI, <span class="literal">true</span>);</span><br><span class="line">ctx.stroke()</span><br></pre></td></tr></table></figure>
<img src="http://blogs-images.youchen-blog.cn/Snipaste_2020-08-31_16-55-41.png" alt="image-20200831165857913" style="zoom:33%;" />

<h4 id="ctx-arc参数说明"><a href="#ctx-arc参数说明" class="headerlink" title="ctx.arc参数说明"></a>ctx.arc参数说明</h4><ul>
<li>圆心横坐标</li>
<li>圆心纵坐标</li>
<li>半径</li>
<li>起始弧度</li>
<li>终止弧度</li>
<li>顺时针还是逆时针画（true代表逆时针）</li>
</ul>
<h4 id="ctx-arc的起始弧度和终止弧度对照图"><a href="#ctx-arc的起始弧度和终止弧度对照图" class="headerlink" title="ctx.arc的起始弧度和终止弧度对照图"></a>ctx.arc的起始弧度和终止弧度对照图</h4><img src="http://blogs-images.youchen-blog.cn/Snipaste_2020-08-31_16-58-36.png"  />

<h3 id="画矩形"><a href="#画矩形" class="headerlink" title="画矩形"></a>画矩形</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> canvas = <span class="built_in">document</span>.getElementById(<span class="string">&#x27;canvas&#x27;</span>)</span><br><span class="line"><span class="keyword">const</span> ctx = canvas.getContext(<span class="string">&#x27;2d&#x27;</span>)</span><br><span class="line">canvas.width = <span class="number">600</span>;</span><br><span class="line">canvas.height = <span class="number">600</span>;</span><br><span class="line"></span><br><span class="line">ctx.strokeRect(<span class="number">300</span>, <span class="number">100</span>, <span class="number">200</span>, <span class="number">100</span>)</span><br></pre></td></tr></table></figure>

<h4 id="strokeRect参数"><a href="#strokeRect参数" class="headerlink" title="strokeRect参数"></a>strokeRect参数</h4><ul>
<li>矩形左上角横坐标</li>
<li>矩形左上角纵坐标</li>
<li>宽</li>
<li>高</li>
</ul>
<p><strong>注：</strong> <code>strokeRect</code>函数将<code>ctx.stoke</code>封装进去了，所以我们不用调用 <code>ctx.stoke</code> 也可以绘制</p>
<h3 id=""><a href="#" class="headerlink" title=""></a></h3>
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